In recent years, the number of wireless communication apparatus that can handle a wireless system of a dual band using two frequency bands of a high band and a low band as typified by a mobile telephone has increased. Among the wireless communication apparatus, to enhance convenience, a wireless communication apparatus incorporating another wireless system such as a wireless LAN also makes its appearance.
As an example, a wireless communication apparatus provided by combining a GSM mobile telephone of a dual band using a 900-MHz band and a 1800-MHz band and a DECT cordless telephone can be pointed out, To use the access line of the DECT cordless telephone as the GSM mobile telephone, it is made possible to use the DECT cordless telephone even in a place where no telephone line exists, and convenience improves.
However, if a wireless system of a dual band and another wireless system are incorporated in one wireless communication apparatus, coupling caused by an antenna current flowing through a board occurs and it becomes impossible to conduct stable communications because of interference depending on the combination.
In the example described above, since the 1800 MHz band of GSM (1710 to 1880 MHz) is adjacent to the DECT band (1880 to 1900 MHz), if a monopole antenna is used as an antenna, interference occurs due to the antenna current flowing into the board and it becomes impossible to conduct stable communications.
If wireless systems having close frequencies are combined, to circumvent interference caused by an antenna current flowing into a board, a dipole antenna where no antenna current flows into the board is effective and hitherto has been used.
Thus, to use a dipole antenna for a dual band antenna of a wireless communication apparatus for making possible the DECT cordless telephone incorporating the GSM mobile telephone as described above, for example, a configuration shown in FIG. 10 is considered in a background art.
FIG. 10 shows a configuration example of a wireless communication apparatus using a background dual band antenna. In FIG. 10, numeral 40 denotes a board. The direction parallel to the board face of the board 40 and orthogonal to left and right side ends is the direction of a horizontal line. This means that the horizontal plane is a plane perpendicular to the board face of the is board 40 and parallel to the top and bottom side ends of the board 40. The direction parallel to the board face of the board 40 and orthogonal to the top and bottom side ends is the direction of a vertical line. This means that the vertical plane is a plane perpendicular to the board face of the board 40 and parallel to the left and right side ends of the board 40.
A wireless circuit of a GSM mobile telephone is placed on the left of the board face of the board 40 and a wireless circuit of a DECT cordless telephone is placed on the right. A ground conductor 39 is provided in the area where they are placed, and necessary connection is made.
The wireless circuit of the GSM mobile telephone includes a dipole antenna 33 of a dual band provided piercing the board face of the board 40 and a GSM module 35 for transmitting and receiving a GSM signal, the dipole antenna and the GSM module connected by a feeder line 34 of a microstrip line. The dipole antenna 33 has a configuration wherein each trap 32 made of a parallel resonant circuit made up of a capacitor and a coil is inserted in a midpoint of a radiation element 31. Putting the dipole antenna into a dual band with traps inserted in a radiation element is a generally adopted technique.
The wireless circuit of the DECT cordless telephone includes a dipole antenna 36 of a single band provided piercing the board face of the board 40 and a DECT module 38 for transmitting and receiving a DECT signal, the dipole antenna and the DECT module connected by a feeder line 37 of a microstrip line.
The dipole antenna 33 and the dipole antenna 36 have the radiation elements placed so that they are inclined 45 degrees with respect to the vertical plane and are orthogonal to each other considering the directivity in the horizontal plane and also considering circumventing of coupling caused by a radiation wave.
It is known that a current flows only into the radiation element in the dipole antenna; while a current paired with a current flowing through a radiation element also flows into a conductor in a monopole antenna. Therefore, according to the configuration shown in FIG. 10, the dipole antenna is used for both the antenna connected to the GSM module and the antenna connected to the DECT module, whereby mutual antenna currents do not flow into the ground conductor and it is made possible to conduct stable communications without causing interference.
Moreover, in recent wireless communications, the case where very close frequency bands are used between different wireless systems has often occurred. Thus, if a highly convenient communication apparatus is configured using two wireless systems in combination, depending on the combination of the wireless systems, they interfere with each other and a problem arises in that the case where stable communications cannot be conducted occurs.
For example, GSM (Global System for Mobile Communications) exists as the standard of a mobile telephone and DECT (Digital Enhanced Cordless Telecommunications) exists as the standard of a cordless telephone. The DECT is a standard for connecting a base unit used in DECT to a public telephone network arriving at each home for use as a cordless telephone. In this case, if the base unit used in DECT is provided with a GSM transmission-reception section for making GSM available and it is made possible to connect the base unit used in DECT to the public telephone network, the cordless telephone can also be used in a place where no telephone line exists or an area where the public telephone network is not built, and convenience for the user is enhanced.
However, DCS1800, one of GSM use bands, is assigned a frequency band of 1710 MHz to 1880 MHz. On the other hand, DECT is assigned a frequency band of 1880 MHz to 1900 MHz. That is, if the DECT base unit is connected to the public telephone network using GSM, since DCS1800 and GSM have adjacent bands, when receiving a signal from a GSM base station, the GSM transmission-reception section of the DECT base unit also receives a transmission signal of the DECT base unit; conversely, when the DECT base unit receives a signal from a DECT cordless handset, the GSM transmission-reception section of the DECT base unit also receives a signal transmitted to a GSM base station, and a problem arises in that it becomes impossible to conduct mutually stable communications.
Therefore, in a communication apparatus provided by combining a plurality of wireless systems using close frequency bands, to circumvent interference of a transmission signal of another wireless system when any desired signal is received, it becomes important to isolate a plurality of antennas in wireless devices. On the other hand, in recent years, it has become hard to sufficiently space installed antennas from each other with miniaturization of a wireless device and thus a new problem also arises in how isolation between the antennas is ensured in a limited space.
As an antenna apparatus adopting a measure to ensure isolation between the antennas in a limited space, for example, an antenna apparatus disclosed in (Patent literature 1) is known. (Patent literature 1) discloses an antenna apparatus wherein two wireless devices housed in the same cabinet use each a monopole antenna, a conductor is placed in the proximity of one antenna, an antenna current of the other antenna is introduced into the conductor, and coupling caused by the antenna current is decreased, whereby isolation between the antennas can be ensured.